Information playback systems frequently utilize a stylus for reading signals from the surface of an information record, typically a plastic disc that contains stored video and audio information. In some systems the information record has a fine spiral groove to guide the tip of a stylus that contains a thin electrode. In these systems, the stylus tip is made of a material having sufficient hardness to withstand the abrasion caused from tracking the groove. Materials which possess such hardness, such as diamond, generally have a crystallographic structure which presents surfaces exhibiting different qualities depending upon which crystallographic plane the surfaces are oriented along. The video disc stylus utilized in the CED (capacitance electronic disc) system is tapered to form the prow of the tip, and is also lapped to form a keel having a V-shaped shoe for its bottom portion. This keel-shaped tip has a shoe length of about 3 to 5 micrometers and a thickness of about 2 micrometers. Making a long-shanked stylus entirely from the same material may become expensive, particularly when the tip material, for example diamond, exceeds the cost of other suitable materials from which the shank can be made.
In order to reduce manufacturing costs, the shank of the stylus may be made from a different material which is less expensive than the crystallographic tip material. For example, a small diamond stone may be mounted at the end of a relatively long metallic shank, such as a cylindrical titanium rod. The diamond stone utilized may be a synthetic diamond stone which is less expensive to obtain than a natural diamond stone. The synthetic diamond stone has a plurality of facets oriented along the {100} family of planes and a plurality of facets oriented along the {111} family of planes. The diamond stone is typically mounted in a concave-shaped cavity at the end of the metallic shank by any means capable of holding the stone therein, such as utilizing a braze, setting the stone in a pocket of adhesive epoxy or the like.
In fabricating the keel-shaped tip from the shank-mounted diamond stone, the end at which the stone is mounted is first coned, in order to form a conical diamond tip and also remove a portion of the surrounding shank metal, so that it will not interfere with subsequent stylus processing. A novel method for performing this coning is described in a commonly-owned patent application of E. F. Cave and J. J. Cowden entitled "STYLUS MANUFACTURING METHOD", RCA Docket No. 76,657, filed on Oct. 13, 1981, and having Ser. No 310,857. This coning method produces a prismatic cone which exhibits fourfold symmetry due to the anisotropic hardness of the diamond crystalline structure. The cone actually becomes a tetrahedron with slightly-rounded sides due to compliance with system parameters. A major advantage of this method is that the crystallographic directions are automatically revealed, thereby providing an alignment means for orienting the stylus during subsequent processing. In order to properly perform the novel coning method, it is necessary that the lapping operation be carried out under critical system specifications, including the application of a substantially constant force of predetermined magnitude between the stylus and rotating scaife. In order to accomplish such a lapping operation, the present invention provides a novel stylus coning apparatus for positioning and holding the tip of a stylus adjacent a rotating scaife while continuously rotating the stylus.